1. Field of the Invention
This invention relates to a liquid level sensor having a float arm swingable in accordance with a liquid level, and more particularly to a liquid level detection apparatus which is suitably mounted on a fuel tank of a vehicle using a liquid (such as gasoline or gas oil) as fuel, and detects a residual amount of the fuel.
2. Related Art
A conventional liquid level detection apparatus, having a structure shown in FIGS. 6 to 9, is commonly used (see JP-A-2004-061420 (FIGS. 8 to 10)).
As shown in FIG. 6, the conventional liquid detection apparatus 91A comprises a resin-molded frame 92, a metallic float arm 93, a resin-molded arm holder 94, contact plates 95, a float which has buoyancy relative to a liquid to be measured, and is mounted at a distal end portion of the float arm 93, and a circuit board 97 having a resistance plate mounted thereon.
One end portion of the float arm 93 which serves as a rotation shaft portion 93a is passed through a mounting hole 94a in the arm holder 94, and is rotatably inserted in amounting hole 92a formed in the frame 92.
A distal end portion of the rotation shaft portion 93a is fitted in an annular projecting portion 96 formed on and projecting downwardly from a lower side of the frame 92. The arm holder 94 is mounted on the frame 92 so as to rotate or swing around an axis of the rotation shaft portion 93a. The float is mounted on the other end portion (that is, the distal end portion) of the float arm 93.
The contact plates 95, fixed to the arm holder 94, have resiliency, and contacts 95, provided respectively at distal end portions of the contact plates 95, are urged by the resiliency into contact respectively with conductor portions 97a (see FIG. 9) mounted on the frame 92.
As shown in FIG. 7, the two contact plates 95 are arranged adjacent to each other in a forward-rearward direction. The first contact plate 95 and the second contact plate 95 are held in contact respectively with the conductor portions 97a provided on the circuit board 97.
As shown in FIG. 8, each contact 95a includes a lower portion 98 of a larger diameter, and an upper portion 99 of a smaller diameter, and an annular groove 100 is formed between the lower portion 98 and the upper portion 99. This annular groove portion 100 is fitted in a hole formed through the contact plate 95. A lower surface of the lower portion 98 is adapted to contact the conductor portion 97a. 
The liquid level detection apparatus 91A is mounted on a container or vessel for storing a liquid, and when the float moves upward or downward in accordance with a change in the liquid level, the float arm 93 is swung around the axis of the mounting hole 92a within a predetermined angle range.
In accordance with this swing movement, the contact 95a on each contact plate 95 slides over the conductor portion 97a, with its lower portion 98 held in sliding contact with the conductor portion 97a, so that a resistance value between one end of the conductor portion 97a and the contact 95a on the contact plate 95 changes in a sensor circuit (not shown). This resistance change is detected by a sensor circuit, thereby obtaining a level signal representative of the amount of change of the liquid level.
As shown in FIG. 9, in the conventional liquid level detection apparatus 91A, when the contact plate 95 is to be mounted on the frame 92, the contact plate 95 is inserted in a direction of an arrow in FIG. 9. In this inserting operation, the contacts 95a, while rubbing an insertion region 99 (indicated by hatching in FIG. 9) of the ceramics board, are inserted respectively into predetermined positions.
In the above conventional liquid level detection apparatus 91A, however, when the contact plates 95 are to be mounted on the frame 92, the contacts, while rubbing the ceramics board, are brought into the predetermined positions because of the structure of the parts. Generally, each contact 95a is made of a silver alloy, and is lower in hardness than a ceramics material, and therefore when the contact rubs the ceramics material, there have been encountered problems that the surface of the contact takes scratches and that the surface of the contact reacts with moisture in the air, and is ionized to be deteriorated (an ion migration phenomenon).
And besides, the scratches on the contact surface temporarily cause a contact failure of the contact, so that the output value becomes unstable. Therefore, the conductor is affected by the contact failure of the contact, and an indication of a fuel gauge becomes inaccurate, which has invited a problem that the operation of the vehicle on which the fuel gauge is mounted is affected.